1. Field of the Invention
The present invention relates to apparatus and methods for remotely detecting small physiological changes in a human and, more specifically, to a phase difference locked loop circuit for out-of-band noise, motion artifacts and temperature drift reduction. More broadly, the invention can be used to reduce distortion and drift in any radar return.
2. Description of the Related Art
There is a need to be able to detect remotely small physiological changes in a human in order to measure the human's condition including physiological stress levels and viability. Measurable indices of autonomic nervous system (ANS) status such as heart rate variability (HRV) have been shown to be altered under stress. Neural inputs to the heart's sinoatrial node accelerate or decelerate the heart rate over varying time scales depending on the respiration (RESP) rate and the balance of tone of the two branches of the ANS. Mental stress has been shown in human studies to increase sympathetic tone, which shifts mean heart rate (HR) higher, and decrease parasympathetic tone, which in addition will reduce the variability of inter-beat intervals. Under a controlled protocol, in which a baseline of ANS activity can be established, shifting of mean HR and HRV can be used to indicate changing ANS state. Unfortunately, small physiological changes can be difficult to detect with sufficient accuracy remotely using radar systems because of non-linear response in derived motion signals and drifting of signal characteristics with temperature due to temperature-dependent properties of associated electronics. What is needed are apparatus and methods to determine inter-beat interval with sufficient fidelity to allow a determination of stress, viability and other human conditions to be performed remotely and accurately.